Micromachined devices

ABSTRACT

A micromachined device has a body suspended over a substrate and movable in a plane relative to the substrate. The body has a perimeter portion, a first cross-piece portion extending from one part of the perimeter portion to another part of the perimeter portion to define at least first and second apertures, a first plurality of fingers extending along parallel axes from the perimeter portion into the first aperture, and a second plurality of fingers extending along parallel axes from the perimeter portion into the second aperture.

CROSS REFERENCED TO RELATED APPLICATION

This application is a divisional of application Ser. No. 10/022,690,filed Dec. 17, 2001, now U.S. Pat. No. 6,505,512, and is a divisional ofapplication Ser. No. 10/022,688, filed Dec. 17, 2001, now U.S. Pat. No.6,481,284, and is a divisional of application Ser. No. 10/022,681, filedDec. 17, 2001, now U.S. Pat. No. 6,487,908; each of which is adivisional of application Ser. No. 09/645,199, filed Aug. 25, 2000, nowU.S. Pat. No. 6,505,511; which is a divisional of application Ser. No.08/921,672, filed Sep. 2, 1997, now U.S. Pat. No. 6,122,961. Theforegoing are all incorporated herein by reference.

BACKGROUND OF THE INVENTION

This invention relates to micromachined gyros.

A surface micromachined gyro has a planar body (or a number of bodies)suspended with anchors and flexures over and parallel to an underlyingsubstrate. The body is dithered along a dither axis in a plane parallelto the substrate and perpendicular to a sensitive axis that can be inthe plane of the body or perpendicular to the body and to the substrate.As is generally known, rotation by the body about the sensitive axiscauses the body move along a Coriolis axis, which is mutually orthogonalto the dither axis and the sensitive axis. This motion can be sensed toderive a signal that indicates the angular velocity of the rotation.

Because of mechanical imperfections in the body and in the flexures, asuspended mass will typically not be perfectly parallel to thesubstrate, and the dither and sensitive will typically not be perfectlyorthogonal. Consequently, when the body is dithered, an interferencesignal, referred to as the quadrature signal, is induced by thedithering motion itself. This quadrature signal, which is unrelated tothe rotation to be sensed, interferes with the desired signal relatingto the rotation. The quadrature signal (a) is proportional to theacceleration in the dither direction with a constant of proportionalityindicative of the mechanical misalignment; (b) has the same frequency asthe dither frequency; and (c) is 90° out of phase with the dithervelocity, unlike the Coriolis signal which is in phase with thevelocity. Because of this 90° phase difference, the quadrature signalcan be partially rejected with a phase-sensitive detector. Theeffectiveness of such rejection, however, depends on how precise thephase relationships are maintained in the electronics.

SUMMARY OF THE INVENTION

In one aspect, the present invention is a micromachined gyro in whichthere is minimal interference in the output signal caused by the dithersignal. The gyro has a first body, suspended over a substrate anddithered along a dither axis, and a second body coupled to the firstbody and also suspended over the substrate. The first and second bodiesare coupled together and anchored to the substrate such that the firstbody can move along the dither axis but is substantially inhibited frommoving along a Coriolis axis (perpendicular to the dither axis) relativeto the second body, and the second body is movable with the first bodyalong the Coriolis axis but is substantially inhibited from moving alongthe dither axis. The coupling between the first body and the second bodysubstantially decouples the dithering movement from the movement alongthe Coriolis axis in response to rotation about the sensitive axis, thusminimizing the unwanted quadrature signal. One of the first and secondbodies preferably surrounds the other; the dithered first body ispreferably on the inside and surrounded by the second body, although thefirst body can surround the second body.

In another aspect, a micromachined device has a first body with fingersinterdigitating with fixed drive fingers that cause the first body todither along a dither axis. At least one conductive member is formedunder some, but not all, of the fixed dither drive fingers and iselectrically coupled to the drive fingers to keep the first body in thedesired vertical plane and to prevent the first body from levitating dueto fringe effects.

In yet another aspect, a micromachined device has a movable bodysuspended over a substrate and at least one stop member positioned nearthe movable body. The stop member includes a hook portion extending overthe movable body such that the stop member limits both lateral movementand vertical movement by the body.

In still another aspect, a micromachined device has a suspended movablebody with an outer perimeter portion and at least one cross-piece thatdefines a number of apertures enclosed by the perimeter portion. Thebody has fingers extending into the apertures. These fingers can be usedeither to dither the body or to sense motion of the body.

In another aspect, the micromachined device has an inner body surroundedby an outer body, the outer and inner bodies being inhibited frommovement together along one axis by flexures oriented along that oneaxis. These flexures are connected between the body and an elongatedstationary member anchored at a midpoint and with the flexures extendingfrom each end to the body. The elongated member is preferably betweenthe inner and outer bodies

In still another aspect of the invention, a first micromachinedstructure is positioned near a second micromachined structure, and thefirst micromachined structure is dithered relative to the secondmicromachined structure. These first and second structures are connectedtogether with coupling structures designed to minimize stress and toencourage opposite ends of the structure to move together in thedirection of dithering toward and away from the second structure. Whilethere are a number of variations of coupling structures that can beused, these include structures that have elongated members extendingfrom ends of the first structure and extending toward the center of thestructure along a direction perpendicular to the dithering direction.These elongated members are connected by a short connecting beam thatencourages the elongated members to move together in the same directionat the same time, rather than moving in opposite directions. Theseelongated members are connected to the first structure withperpendicular members that define a pivot point.

Openings can be cut out of the second structure to reduce the combinedmass of the first and second structures, while still maintainingstiffness in the structure. In addition to the coupling structuresbetween the first and second structures, the second structure is alsoanchored to the substrate through plates that are relatively widecompared to the width of the second structure itself. These plates areconnected together by perpendicular members that define a pivot point.

The gyro of the present invention minimizes the quadrature signal, andthus is very accurate compared to prior gyro designs. The accuracy dueto the structure of the gyro obviates the need for complex electronics,and also allows the device to be packaged under ambient conditions. Thestop members, the conductive members on the substrate, use of multipleapertures with inwardly extending fingers, and use of a centrallyanchored stationary member for supporting flexures along an axis ofinhibited movement all improve performance and reliability of amicromachined device in general and a gyro in particular. Other featuresand advantages will become apparent from the following detaileddescription, the drawings, and the claims.

BRIEF DESCRIPTION THE DRAWINGS

FIG. 1 is a plan view of a micromachined gyro that is similar inprinciple to known gyros.

FIG. 2 is a plan view of a micromachined gyro according to a firstembodiment of the present invention.

FIG. 3 is a plan view of a gyro according to a second embodiment of thepresent invention.

FIG. 4 is a more detailed plan view of a portion of a gyro according toa third embodiment of the present invention.

FIGS. 5-7 are cross-sectional views of portions of the gyro of FIG. 4illustrating certain features in the gyro.

FIG. 8 is a partial pictorial and partial block diagram of a circuit foruse with a gyro of the type shown in FIGS. 2 and 4.

FIG. 9 is a cross-section view of a frame illustrating a desiredplacement of the anchors.

FIG. 10 is a plan view illustrating a gyro according to a fourthembodiment of the present invention.

FIG. 11 is a plan view illustrating a gyro with fixed fingers serving toform one electrode on each side of two spaced rows of capacitive cells.

FIG. 12 is a plan view of a portion of a gyro showing a connection froman inner frame to an outer frame according to another embodiment of thepresent invention.

FIG. 13 is another plan view of the gyro of FIG. 12, illustrating theforces on the gyro during dithering.

FIGS. 14 and 15 are plan views illustrating yet another embodiment ofthe present invention.

DETAILED DESCRIPTION

FIG. 1 illustrates a simplified surface micromachined gyro 10 that isstructurally and operationally similar to known gyros. Gyro 10 has anessentially planar body 12 suspended over and parallel to an underlyingsubstrate 14. Body 12 is supported with four flexures 16, each of whichextends from a respective support anchor 18 to a different corner ofbody 12. Fingers 22 extend from body 12 and interdigitate with fixeddrive fingers 24 that are coupled to an AC voltage source (not shown) todither the body at its resonant frequency along a dither axis 26. Whenbody 12 rotates about a sensitive axis 36, body 12 moves along aCoriolis axis 28 that is mutually orthogonal to sensitive axis 36 anddither axis 26. This movement is sensed with a differential capacitorthat includes fingers 30 that extend away from body 12 along axesparallel to dither axis 26, and two sets of inwardly extending fixedsensing fingers 32 and 33. The differential capacitor is formed frommany individual cells, each cell having two fixed fingers 32, 33 and onefinger 30 serving as a movable finger and interdigitation with fixedfingers 32, 33.

As is generally known, if the dither motion is x=×sin(wt), the dithervelocity is x′=w×cos(wt), where w is the angular frequency and isdirectly proportional to the resonant frequency of the body by a factorof 2π. In response to an angular rate of motion R about sensitive axis36, a Coriolis acceleration y″=2Rx′ is induced along Coriolis axis 28.The signal of the acceleration thus has the same angular frequency w asdithering velocity x′. By sensing acceleration along Coriolis axis 28,rotational velocity R can thus be determined.

Due to mechanical imperfections, e.g., if one flexure is more compliantthan the others due to overetching, the center of suspension of the bodymay not coincide with its center of mass and thus the mass can wobbleduring the dithering motion. Such wobbling causes a component of thedither motion to appear along the sensitive axis. This component createsthe interfering quadrature signal. This signal can be very largecompared with the desired rotational signal being measured; e.g., it canbe as much as 10% of the dither motion, creating a signal 10,000 timesgreater than the Coriolis signal. The need to eliminate this quadraturesignal places a great burden on the signal processing electronics.

FIG. 2 is a plan view of a surface micromachined gyro 50 illustrating asimplified first embodiment of the present invention. Gyro 50 has asuspended body with an inner frame 52 and an outer frame 54 surroundinginner frame 52. Frames 52, 54 are coplanar and are suspended over andparallel to an underlying substrate 55. Outer frame 54 is suspended withflexures 56 that extend along axes parallel to a dither axis 58 and areanchored to substrate 55 with anchors 53. This orientation of flexures56 allows outer frame 54 to move along a Coriolis axis 60, butsubstantially prevents outer frame 54 from moving along dither axis 58.

Inner frame 52 is coupled to and suspended from outer frame 54 withflexures 62 that extend along axes parallel to Coriolis axis 60. Theorientation of flexures 62 allows inner frame 52 to move along ditheraxis 58 relative to outer frame 54, substantially inhibits relativemotion of the frames along Coriolis axis 60, but allows inner frame 52and outer frame 54 to move together along Coriolis axis 60. Accordingly,for both inner frame 52 and the outer frame 54, control over allowableand inhibited directions of movement is achieved by orienting the axesof the flexures along the inhibited axis.

To summarize these allowed and inhibited movements, inner frame 52:

(a) can move relative to outer frame 54 along the dither axisindependent of the movement of outer frame 54; and

(b) cannot move along the Coriolis axis relative to outer frame 54, butcan move along the Coriolis axis with outer frame 54;

while outer frame 54:

(a) cannot move along the dither axis; and

(b) can move along the Coriolis axis but only by moving inner frame 52along with it.

Anchors 53 for flexures 56 are preferably located within the spacebetween inner frame 52 and outer frame 54. Referring to FIG. 9, thisstructure is useful because a suspended structure, such as outer frame54, can tend to have a bowed shape with a high point in the center andlow points at the ends. It is desirable for the anchors to be located atthe vertical position of the center of gravity 57 so that the extremesof bowing do not unmesh the fingers and so that any wobbling induced bythe center of gravity being vertically displaced from the center ofsuspension is minimized.

Referring again to FIG. 2, inner frame 52 is shaped generally as arectangular annulus with a central rectangular aperture 64. Drivefingers 66 and sensing fingers 67 extend inwardly from inner frame 52into aperture 64 along parallel axes that are parallel to dither axis58. Positioning drive fingers 66 in the aperture as shown helps tomaximize the outer perimeter and area of inner frame 52, thus allowingfor a larger numbers of drive fingers, thereby improving the response tothe dither signal. Drive fingers 66 interdigitate with fixed ditherdrive fingers 68, while sensing fingers 67 interdigitate with fixedsensing fingers 69. Fixed fingers 68 and 69 are anchors to and fixedrelative to underlying substrate 55, while fingers 66 and 67 move withinner frame 52 and thus are movable relative to substrate 55.

A drive signal is provided from a dither drive mechanism (not shown)that includes an AC voltage source coupled to fixed dither drive fingers68 to cause inner frame 52 to be dithered relative to outer frame 54along dither axis 58 at a velocity such as x′=w×cos(wt) as noted above;more preferably, the dither is caused by a square wave. The ditheringmotion is sensed by the change in capacitance between movable sensingfingers 67 and fixed sensing fingers 69. This sensed motion is amplifiedand fed back to the dither drive mechanism to sustain the ditheringmotion at the resonant frequency of the inner frame.

When there is no rotational velocity R about sensitive axis 64, outerframe 54 does not move relative to substrate 55. When there is arotational velocity R, inner frame 52 will tend to move along Coriolisaxis 60 with an acceleration y″=2Rx′, which is2(R/w)x″(cos(wt)/sin(wt)), because x″=w²×sin(wt). Flexures 56 allow theinner frame 52 to move with outer frame 54 along the Coriolis axis. Notethat the ratio of y″ to x″ is 2R/w, which is actually modified by m/M,where m is the mass of the inner frame, and M is the total mass of bothframes. Assuming m/M=1/2, R=1 rad/sec, and w=2π×10⁴ rad/sec, the ratioof the magnitudes of y″ to x″ is about 16 ppm.

To sense the movement along Coriolis axis 60, outer frame 54 has fingers70 that extend along axes parallel to the dither axis and interdigitatewith fixed fingers 72, 74 on either side of fingers 70 (fingers 72, 74are only shown on one side). Fingers 72, 74 are fixed with anchors 73 tosubstrate 55. Fingers 72 are electrically connected together to a firstfixed DC voltage source with a voltage V₁, and fingers 74 are connectedtogether to a second fixed DC voltage source with a voltage V₂. Asfingers 70 of outer frame 54 move toward one or the other of fingers 72or 74, the voltage on outer frame 54 changes. By sensing the voltage onouter frame 54, the size and direction of movement can therefore bedetermined.

If desired, a carrier signal with a frequency much larger than thedither frequency can be applied to fixed fingers 72, 74, and theresulting output is then amplified and demodulated. Such sensingtechniques are known in the field of linear accelerometers. A carriersignal is not necessary with the structure of the present invention,however, because this structure substantially eliminates the interferingquadrature signal, and thus the added complexity in the circuitry isundesirable if avoidable.

Referring to FIG. 3, in another embodiment of the present invention, onelarge gyro 150 includes four substantially identical gyros 152 a-152 darranged in a rectangular configuration and shown here in a simplifiedform. Gyros 152 a-152 d have respective inner frames 154 a-154 d, outerframes 156 a-156 d, dither drive structures 158 a-158 d and 159 a-159 don opposite sides of the inner frames, dither sensing structures 160a-160 d and 161 a-161 d, and fixed fingers 162 a-162 d and 164 a-164 dfor sensing motion along the Coriolis axes. These gyros are connected ina “cross-quad” manner as shown. With this interconnection, fixed fingers162 a and 162 c are electrically connected together and to fixed fingers164 b and 164 d; fixed fingers 162 b and 162 d are electrically coupledtogether and to fixed fingers 164 a and 164 c; dither drive structures158 a, 158 c, 159 b, and 159 d are electrically connected together;dither drive structures 158 b, 158 d, 159 a, and 159 c are electricallyconnected together; dither sensing structures 160 a, 160 c, 161 b, and161 d are electrically connected together; and dither sensing structures160 b, 160 d, 161 a, and 161 c are electrically connected together.

Such a cross-quad connection eliminates errors due to manufacturing andtemperative gradients and also eliminates sensitivity to external linearacceleration. Such a connection is also described in Patent PublicationNo. WO 96/39615, which is expressly incorporated by reference for allpurposes.

FIG. 4 is a detailed view of a little more than one-half of one gyro 80;the other half of gyro 80 is substantially the same as the half that isshown. As in the embodiment of FIG. 2, each gyro has an inner frame 82and an outer frame 88. Flexures 90 extend along axes parallel to aCoriolis axis 86 from inner frame 82 to outer frame 88. With theseflexures, inner frame 82 can move along a dither axis 84 relative to anouter frame 88, but is substantially inhibited from moving alongCoriolis axis 86 relative to outer frame 88. Outer frame 88 is movablealong Coriolis axis 86 along with inner frame 82.

The structures have a number of larger openings 146 in the inner andouter frames resulting from the removal of pedestals made of photoresistand later etched away as part of the manufacturing process. Smallerholes 148 are formed in the structures so that a solvent can beintroduced to etch out a sacrificial oxide layer. Such processingtechniques for surface micromachined accelerometers are generally knownand are described, for example, in U.S. Pat. No. 5,326,726, which isexpressly incorporated by reference for all purposes.

Inner frame 82 is roughly shaped as a rectangular ring with tworelatively long sides 92 (one of which is shown) and two relativelyshort sides 94. Extending along the interior aperture surrounded byframe 82 are two elongated cross-pieces 96 (one of which is shown)integrally formed with the outer ring of inner frame 82 and extendingparallel to relatively long sides 92 from one relatively short side tothe other. Inner frame 82 thus has three elongated apertures 98 (one anda half of which are shown), rather than the one shown in the embodimentof FIG. 2. With these multiple apertures, there can be additional rows(six in this case) of movable dither fingers and fixed dither fingersinstead of two, thus increasing response and consistency.

Extending into apertures 98 from both long sides 92 and from elongatedcross-pieces 96 are drive fingers 100 and sensing fingers 102 extendingin parallel and along axes parallel to dither axis 84. Drive fingers 100and sensing fingers 102 interdigitate with fixed drive fingers 104 andwith fixed dither sensing fingers 106, respectively. Fixed drive fingers104 are driven with an AC signal to cause drive fingers 100, and henceinner frame 82, to move along dither axis 84. If the AC signal issinusoidal, the inner frame moves with a displacement x=X sin(wt), andtherefore with a velocity of x′=w×cos(wt), with angular frequencyw=2πf_(res) (resonant frequency f_(res) can be different for differenttypes of structures, but is typically in the 10-25 KHz range).

Fixed dither sensing fingers 106 interdigitate with fingers 102, and thechange in capacitance between these fingers is sensed to monitor thedither motion and to provide a feedback signal to the dither drive tomaintain the dither motion at the desired angular frequency w. Fixedsensing fingers 106 are anchored to substrate 98 with anchors 130 andare electrically coupled together with conductive lines 132 formed onsubstrate 98. Fixed dither drive fingers 104 are anchored to substrate98 with anchors 134 and are electrically coupled away from the gyro withconductive lines 136.

Along relatively short sides 92 between inner frame 82 and outer frame88 are two stationary members 110 anchored to and fixed relative tosubstrate 98. Flexures 114 extend from outer frame 88 to stationarymembers 110 along axes parallel to dither axis 84, and thereforesubstantially prevent outer frame 88 from moving along dither axis 84.Stationary members 110 are anchored to substrate 98 with anchors 109that are located at the midpoint of stationary members 110. Thislocation minimizes stress because any shrinkage that occurs instationary members 110 and flexures 114 during manufacturing is similarto that in outer frame 88. Therefore, there is no residual stress in thedither direction in flexures 114.

Stationary members 110 are very useful because they provide for flexures114 to have the correct length, provide attachment points for flexures114 that are far from a center line of the device, thereby stabilizingouter frame 86 against tilting, and provide freedom from shrinkage alongthe lengthwise direction of flexures 114.

Extending outwardly away from outer frame 88 along axes parallel todither axis 84 is a large number of fingers 111, each of which isdisposed between two fixed sensing fingers 112, 113 to form a capacitivecell. The large number of cells together form a differential capacitor.Fixed sensing fingers 112, 113 are anchored at their ends and areelectrically connected to other respective fingers 112, 113 and to adifferent DC voltage as noted in connection with FIG. 2.

This assembly of sensing fingers on a suspended frame essentially formsa sensitive accelerometer of the type disclosed in the incorporatedpatent publication, with its function being to sense the Coriolisacceleration. The accelerometer is also sensitive to externally appliedaccelerations, but two of the gyros in the cross-quad arrangement aresensitive in the opposite sense to the other two, thereby canceling suchexternal interference.

In one exemplary embodiment, inner frame 82 and outer frame 88 are eachat 12 volts DC, while fixed fingers 112, 113 are all at 0 volts DC. Asouter body 88 and its fingers 111 move, a change in voltage is inducedon fingers 112, 113. A high frequency carrier signal can be provided tothe fixed sensing fingers, but with the accuracy of the gyro accordingto the present invention, the carrier is not needed, and thus therequired circuitry is minimized by avoiding the need for a highfrequency demodulator.

If there is rotation about a sensitive axis 130 (which is mutuallyorthogonal to both dither axis 84 and Coriolis axis 86), outer frame 86and inner frame 82 move together along Coriolis axis 86 in response tothe rotation. If there is no such rotation about sensitive axis 130, thedither motion of inner frame 82 causes substantially no motion by outerframe 88 along Coriolis axis 86.

The decoupling of motion along the dither axis and sensitive axis hassignificant beneficial effects. Imbalances in the flexures produce verylittle dither motion along the sensitive axis. In this case, theinterfering quadrature signal can be reduced to as low as 0.5 parts permillion (ppm) or 0.00005%; this small quadrature signal results from thesame types of mechanical imbalances that otherwise could produce a 10%interference signal in a gyro of the type generally shown in FIG. 1.Moreover, the rotationally induced acceleration that the gyro isdesigned to sense is inhibited very little. Because of this accuracy,the circuitry need not be particularly complex.

Another benefit from this structure arises in the packaging. Theincrease in the signal from the large number of fingers due to theapertures, and from the four gyros in the cross-quad arrangement,eliminates the need to enhance the signal by reducing air damping, andthus makes ambient packaging possible, rather than more costly vacuumpackaging.

Referring also to FIGS. 5 and 6, another aspect of the present inventionis illustrated. Along much of the row of fixed drive FIGS. 104 areconductive members 126 at voltage V, preferably the same DC voltage asinner frame 82 and outer frame 88 (i.e., 12 volts). Meanwhile, the drivefingers are preferably driven with a square wave with an amplitude of 12volts. At several other locations along the row of fixed dither drivefingers 104, conductive members 120 are formed on substrate 98 undergroups of fingers and are electrically coupled to drive fingers 104. Asshown in FIG. 4, conductive members 120 have a length coextensive withthe length of fingers 104 and a width that extends across five fingers104, while conductive members 126 extend across the inner frame with awidth that extends along fourteen fingers 104.

Referring to FIG. 5, where movable drive fingers 100 and fixed drivefingers 104 are formed over conductive members 126, there will be a netupward force on movable drive fingers 100 due to fringe effects fromadjacent fingers 104, and thus fingers 100 will have a tendency tolevitate. Conductive members 126 are used and kept at 12 volts toprevent static collapse and makes the stray capacitance well-defined.

As shown in FIG. 6, however, where conductive members 120 are formedunder drive fingers 100 and fixed dither fingers 104, an attraction bymovable drive fingers 100 toward substrate 98 causes a net downwardforce that should counteract the net upward force shown in FIG. 5. Thedownward force due conductive members 120 is greater per finger 100 thanthe net upward force shown in FIG. 5 per finger 100 because conductivemembers 120 are formed under fewer fingers. By positioninganti-levitating conductive members 120 periodically along the length,levitation is prevented.

Referring to FIGS. 4 and 7, another aspect of the present invention isillustrated. Gyro 80 in FIG. 4 has four stop members 140 (two of whichare shown) positioned relative to substrate 98 and to inner frame 82 toprevent excessive movement in any direction. Stop members 140 have afirst portion 144 that is substantially coplanar with inner frame 82,and a hook portion 146 that extends over frame 82. Stop member 140 isconnected with an anchor 142 to substrate 98. Frame 82 is substantiallyinhibited from movement both into the stop member in the plane of frame82 due to coplanar portion 144, and also is inhibited from moving toofar upwardly due to hook portion 146.

As described in incorporated U.S. Pat. No. 5,326,726, to producesuspended inner frame 82, a layer of polysilicon is formed over asacrificial oxide. When the oxide is removed (etched), a suspendedpolysilicon structure is left behind. To form stop members 140, afurther oxide layer is formed over inner frame 82, and then a materialfor forming stop members 140 is formed over that further oxide atlocations 147. Etching out this further oxide leaves behind stop members140. The material used for stop members 140 preferably is one thatminimizes the risk of inner frame 82 contacting and sticking to stopmember 140 (a problem referred to as “stiction”). The preferred materialis titanium tungsten (TiW) because this material has low stiction,compatibility with electronics processing, good conductivity, and highmechanical strength. An appropriately coated silicon could also be used.

Referring to FIG. 8, a circuit is shown for use with gyros such as thoseshown in FIGS. 2 and 4. In FIG. 8, a gyro body 200 includes both a firstbody and a second body interconnected to decouple the dither motion fromthe Coriolis motion. Body 200 is maintained at an elevated voltagerelative to sensing plates 204, and is driven with a signal from ditherdrive plates 202 to create a dither motion that is sensed by dithersensing plates 204. The motion along a Coriolis axis is sensed byCoriolis plates 206. This circuitry would be considered rather simple inthat it has a relaxed phase specification, and is made possible by thedesign of the body that substantially eliminates the quadrature signal.

Capacitive sensing plates 204 are coupled to inputs of an amplifier 210that provides two outputs 211, each of which is coupled to inputs ofamplifier 210 through a feedback impedance network Z1, Z2 that isprimarily resistive. The output of amplified 210 is provided to a secondamplifier 212 that provides two outputs along two paths. The first paths214, 216 provide the feedback signal to dither drive plates 202 to helpkeep the body 200 dithering at the resonant frequency. The other twopaths 218, 220 from amplifier 212 are provided to a two pole, doublethrow analog switch 222 that serves as a synchronous rectifier. Switch222 also receives two inputs from the output of an amplifier 224 thatreceives inputs from Coriolis sensing plates 206. Amplifier 224 hasfeedback networks Z3 and Z4 that are primarily capacitive. The signalsfrom amplifier 212 alternate the polarity of the Coriolis signals fromamplifier 224, thereby phase demodulating the Coriolis signals. Theoutput from switch 222 is provided to a buffering low pass filter 230.

FIG. 10 illustrates a simplified plan view of a gyro 250 according toanother embodiment of the present invention. Gyro 250 has an outer frame252 and an inner frame 254. A dither drive mechanism 256 can bepositioned to apply a dithering motion to outer frame 252 throughfingers 260 extending from outer frame 252 parallel to a dither axis262. Inner frame 254 is coupled to outer frame 252 through flexures 270oriented in parallel to a Coriolis axis 272 that is perpendicular todither axis 262. Elongated stationary members 268 extend along ditheraxis 262 and are centrally anchored to the underlying substrate 264through anchors 269. Flexures 266 extend from each end of each anchoredstationary member 268 in a direction parallel to dither axis 262.Flexures 266 thus prevent outer frame 252 from moving along dither axis262, while flexures 270 allow outer frame 252 and inner frame 254 tomove together along Coriolis axis 272. As noted above, stationarymembers 268 control stress and tilt and help keep the flexures at theirappropriate length.

In response to rotation about a sensitive axis 276 (which is mutuallyorthogonal to axes 262 and 272), outer frame 252 and inner frame 254move along Coriolis axis 272. Inner frame has sensing fingers 278extending inwardly into an aperture 280, each located between two fixedfingers 282 such that fingers 278 and fingers 282 form a differentialcapacitor with a number of individual cells. The voltage on inner frame254 can be sensed to determine the change in motion, which, as notedabove, indicates the rotational velocity about axis 276. As in FIG. 3,four gyros of the type shown in FIG. 10 can be connected together in across-quad manner. Moreover, other features discussed above, such as thestop members, positioning of anchors, and conductive members on thesubstrate can be employed with this embodiment of FIG. 10.

Referring to FIG. 11, another improvement is illustrated. In thesituation in which a number of movable fingers are between two sets offixed fingers to make up capacitive cells, one or both of the fixedfingers can be arranged to extend across two gyros or two sets offingers to reduce space and reduce processing. As shown in simplifiedFIG. 11, movable masses 280 and 282 are each movable along the directionof arrows 284 and 286. Each of these masses has respective fingers 288and 290 that move with the respective mass. Fingers 288 and 290 arebetween two stationary fingers, including first fingers 292 and secondfingers 294. As shown here, fixed fingers 292 are formed insubstantially straight lines to form one electrode of the differentialcapacitor with movable finger 288, and also to form one electrode of adifferential capacitor with a movable finger 290. Fixed fingers 292 areformed with a dog-leg configuration so that they extend from one side ofeach movable finger 288 to another side of each movable finger 290 (withthe sides being in reference to the direction indicated by arrows 284and 286). With this arrangement, fewer separate fingers need to bemanufactured, and fewer connections need to be made to the stationaryfingers. Electrical contact points 296, 298 to fingers 292 and 294,respectively, are offset along a direction perpendicular to thedirection of arrows 284 and 286 so that contacts can be made in astraight line with conductors on the surface of the substrate andanchors at the contact points to the conductors on the substrate.

The arrangement shown in FIG. 11 can be used when there are multipleadjacent gyros, such as in the situation illustrated by FIG. 3, and asthe connections would be made in FIG. 4 with multiple gyros. Indeed, inFIG. 4, the connections to fixed fingers 112 and 113 are arranged insuch a staggered fashion, but there is no gyro shown to the side of thegyro in FIG. 4. The arrangement of fixed fingers as shown in FIG. 11could be used in the aperture region of the inner frame in FIG. 10. Byarranging the fixed fingers in this manner, processing is reduced as thenumber of fingers to be formed is reduced, and also space can be madefor additional cells, thereby increasing the signal that is received andimproving accuracy.

Referring to FIG. 12, in another embodiment of the present invention, aportion of a gyro 300 is shown. As shown in FIGS. 4 and 10, gyro 300 hasan inner frame 302, an outer frame 304, anchored stationary beams 306between the inner and outer frames and flexures 308 for preventing theouter frame from moving along a dither axis 310, which is parallel tothe elongated direction of flexures 308. Inner frame 302 is ditheredalong dither axis 310 relative to outer frame 304, which is inhibitedfrom moving along dither axis 310.

In the embodiment of FIG. 4, flexures 90 were oriented perpendicular tothe dither axis for allowing movement along the dither axis. With such astructure, these flexures are under a high tensile force and have atendency to stretch. If significant enough, such stresses could start tobuckle the frame and/or could change the resonant frequency of thesystem.

Referring to FIG. 12, the connection between inner frame 302 and outerframe 304 is made through a connection structure that includes pivotingbeams 312 and 314, that are connected to inner frame 302 throughflexures 316 and 318, and to outer frame 304 with flexures 320 and 322.Pivoting beams 312 and 314 are connected together with a smallcross-piece 324.

Referring to FIG. 13, a close-up and simplified view of the connectionstructure is shown. As inner frame 302 is dithered along dither axis310, inner frame 302 moves as indicated by arrows 330, causingperpendicular stresses along flexures 316 and 318 in the directionindicated by arrows 332. Because pivoting beams 312 and 314 areconnected to flexures 320 and 322, each of which is oriented along adirection parallel to arrows 332, the intersection of beam 312 andflexure 320 and the intersection of beam 314 and flexure 322 form pivotpoints 336 and 338, respectively, causing beams 312 and 314 to movealong the direction indicated by arrows 340 and 342, respectively. Thismovement of the pivoting beams causes a small movement by cross-piece324 along the direction indicated by arrow 344, which is parallel todither axis 310.

This structure encourages movement of beams 312 and 314 in an oppositerotational direction while discouraging simultaneous rotation in thesame direction; i.e., the structure allows anti-phase movement, andsubstantially inhibits in-phase movement. If the pivoting beams were totry to rotate in the same direction at the same time, the cross-piecewould need to lengthen and would undergo a complex twisting motion.Consequently, this structure helps to prevent such movement. Thepivoting mechanism thereby prevents the unwanted motion of dither frame302 perpendicularly to the preferred dither axis, i.e., from producing amotion which interferes with the Coriolis signals. By alleviating thetensile forces in flexures 316, 318, frame 302 can move more freelyalong dither axis 310 and produce a larger signal. The alleviation ofthese tensile forces also prevents distortions of the accelerometerframe by the dither motion, while such distortions could otherwiseproduce interfering signals if unchecked.

In the embodiment of FIGS. 12 and 13, outer frame 308 is shown with alinear inner edge 350 that faces the connection structure and innerframe 302. As an alternative, a portion of outer frame 308 may berecessed relative to edge 350 for connection to flexures 320 and 322.Regardless of the recess, it is desirable for cross-piece 324 to be in aline with flexures 320 and 322.

FIGS. 12 and 13 each show flexures 316 and 318 extending to a corner ofbeams 312 and 314, effectively forming a linear and continuous edge withbeams 312 and 314. To create more space between beams 312, 314 and innerframe 302 when beams 312, 314 pivot, it can be desirable to shave offportions of the edges of beams 312 and 314 facing inner frame 302,particularly at the corner most remote from respective pivot point 336and 338.

Referring again to FIG. 12, there is a difference in the arrangement ofthe apertures and fingers relative to the embodiment of FIG. 4. As shownin FIG. 4, there are three elongated apertures with fingers, and each ofthe apertures has some drive fingers and some pickoff fingers. FIG. 12,by contrast, shows one aperture out of five, and that aperture has onlydrive fingers connected together. In this embodiment of FIG. 12, thereare five apertures, the middle of which is used only for pickoff fingersand not driving fingers, while the other four apertures have onlydriving fingers and not pickoff fingers.

Another difference with respect to the embodiment of FIG. 4 is that inthe embodiment of FIG. 12, the connectors that are used to drive andpick off combs are made from polysilicon formed on the surface of thesubstrate, rather than diffused n+ connectors. With the polysilicon onthe surface, the fingers can be made more accurately, thus allowing morefingers in the same space and therefore more force per unit area.

Another embodiment of the present invention is illustrated in FIGS. 14and 15. A gyro 400 has an inner frame 402 surrounded by an outer frame404. Inner frame 402 is dithered along a dither axis 410 through the useof a dither drive mechanism 406. As described in the embodiments above,dither drive mechanism 406 is preferably formed with combs of drivefingers that interdigitate with fingers on inner frame 402 and aredriven with voltage signals to produce the sinusoidal motion. In theembodiment of FIG. 14, inner frame 402 has four elongated and parallelapertures that include the drive fingers.

In the four corners of inner frame 402 are apertures that have ditherpick-off fingers for sensing the dithering motion. As discussed inembodiments above, this sensed dithering motion is fed back to thedither drive mechanism that drives inner frame 402 along dither axis410.

In response to an angular velocity about a central sensitive axis 412,outer frame 404 is caused to move along a Coriolis axis 414. Asdescribed above, inner frame 402 can be dithered relative to outer frame404, while inner frame 402 is coupled to outer frame 404 so that innerframe 402 and outer frame 404 move together along Coriolis axis 414. Inthis embodiment, the coupling between inner frame 402 and outer frame404, and the anchoring of outer frame 404 to the substrate are designedto improve performance and to reduce the interfering quadrature signalto produce a very high performance gyro.

The couplings are shown in more detail in FIG. 15, which showsone-quarter of gyro 400. The other three quarters of the gyro aresubstantially identical to the portion shown. A dither flexure mechanism430 is coupled between inner frame 402 and outer frame 404 to allowinner frame 402 to move along dither axis 410, but to prevent innerframe 402 from moving along Coriolis axis 414 relative to outer frame404, but rather to move along Coriolis axis 414 only with outer frame404.

FIG. 15 shows half of one dither flexure mechanism 430, which has adither lever arm 432 connected to outer frame 404 through a dither mainflexure 434, and connected to inner frame 402 through pivot flexures 436and 438. Identical components would be on the other side of dashed line442 connected through a small central beam 440 to lever arm 432. Similarto the embodiment of FIG. 12, central beam 440 encourages lever arm 432and the corresponding lever arm connected on the other side of beam 440to move in the same direction along dither axis 410. At the other end oflever arm 432, flexures 436 and 438 extend toward inner frame 402 atright angles to each other to create a pivot point near the junction offlexures 436 and 438.

This coupling and connection mechanism has a number of advantages overother structures recited herein. Because the length of the lever armfrom the pivot point to the small central flexure is long relative tothe total length of the inner frame, the ratio of stiffness of themechanism for perpendicular motion of the dither mass and for relief oftension in the main dither flexure is increased. For a given residualtension, there is good resistance to perpendicular motion, or for agiven resistance, the perpendicular motion creates less distortion inthe accelerometer frame compared to the embodiments of FIG. 12. Flexures436 and 438 can be made long, thereby reducing tension for a givendither displacement. The flexures 436 and 438 are connected to innerframe 402 at points nearer to the center of the inner frame in thelength and width directions, the distortion for a given amount oftension is reduced relative to other embodiments. Because the twopivoting flexures are perpendicular to each other, the pivot point isbetter stabilized than in other structures. By moving the effectiveattachment point of the dither mechanism to the inner frame toward theoutside of the inner frame, there is better stability in the inner frameagainst tilting. To keep lever arm 432 stiff compared to central beam440, lever arm 432 is made wide; while this greater width does improvestiffness, it has the drawback in requiring additional space.

Compared to the embodiment of FIG. 12, outer frame 404 is made stifferby increasing its width. As indicated above, however, the ratio ofsignals is modified by m/M, where M is the total mass of both frames,and m is the mass of the inner frame. Consequently, it is desirable toreduce the mass of the outer frame, so that M is as small as possiblerelative to m. Consequently, a number of holes 444 are cut out of outerframe 404. While the existence of holes 444 reduces the mass, they donot have any substantial effect on the stiffness because they create, ineffect, a number of connected I-beams. This increase in stiffness andperformance does come at the price of increased size of the device,however, thereby lowering yield on the wafer level.

To further improve the performance of device 400 compared to priorembodiments, outer frame 404 is coupled and anchored to the substratethrough a connection mechanism 450 and a pair of anchors 452 that areconnected together. Connection mechanism 450 includes plates 453 and 454connected together with short flexures 456 and 458, which areperpendicular to each other.

The structural polysilicon used to make the masses and flexures shouldbe somewhat tensile in comparison with the substrate so that thestructures fabricated from the polysilicon have a well-defined andsingular form. A consequence of this is that the accelerometer flexuresare slightly bent as manufactured. If these flexures are imbalanced interms of stiffness, a slight static tilt can be introduced into theoverall structure with consequences similar to a tilt of the normalmode. The forces from the dither motion can differentially straightenthe flexures, thereby providing an additional dynamic tilt around thegyro axis. Moreover, the frame bows in response to the tension in thedither flexures, thereby giving a similar effect as does differentialstretching of the flexures from the reaction forces which tilt thestructure in the plane of the substrate.

In the embodiment of FIG. 15, the pivot points are defined by flexures456 and 458 so that outer frame 404 can easily move perpendicular to thedither motion by pivoting plate 453 relative to plate 454 thereby givinga single bending action to flexures 456 and 458 at the ends and in thecenter. To accomplish this, center beam 440 should be co-linear with thepivot points.

Having described embodiments of the present invention, it should beapparent that modifications can be made without departing from the scopeof the invention as defined by the appended claims. While a cross-quadarrangement with four gyros has a number of benefits described above,with highly accurate processing, one may only use two to eliminatecommon mode external accelerations. In yet another alternative, a largerarray with more than four gyros could be arranged and coupled together.

What is claimed is:
 1. A micromachined device comprising: a substrate; afirst body suspended in a plane over the substrate and movable in theplane along a first direction; an anchored member, elongated next to aside of the first body along a second direction transverse to the firstdirection, the anchored member surrounded by the first body, theanchored member having first and second opposite ends in the lengthwisedirection and having flexures extending from each end of the anchoredmember to the first body, the flexures allowing the first body to movein the first direction more easily than in the second direction, themember being anchored with at least one anchor positioned centrallyalong the anchored member between the first and second opposite ends. 2.The device of claim 1, further comprising a set of fingers fixedrelative to the substrate, the first body including a plurality offingers extending in the second direction and interdigitating with thefixed fingers such that movement of the fingers on the first body causesa change in capacitance between the plurality of fingers on the firstbody and the fixed fingers.
 3. The device of claim 1, further comprisinga package, wherein the device is packaged in the package under ambientconditions.
 4. The device of claim 1, wherein the first body issymmetric about a center line parallel to the first direction andincludes sides that extend generally along the first direction, theflexures being coupled to the sides of the first body, the attachmentpoints between the flexures and the anchored member being closer to thesides than to the center line.
 5. The device of claim 1, wherein thefirst body has fingers that are movable with the first body, the devicefurther comprising stationary fingers that form a differential capacitorwith the moving fingers.
 6. The device of claim 1, wherein the firstbody is next to a first side, the device further comprising a secondanchored member elongated along a second side of the first body, thesecond anchored member surrounded by the first body, the anchored memberhaving first and second opposite ends in the lengthwise direction andhaving flexures extending from each end of the anchored member to thefirst body, the flexures allowing the first body to move in the firstdirection more easily than in the second direction, the member beinganchored with at least one anchor positioned centrally along theanchored member between the first and second opposite ends.
 7. Thedevice of claim 6, wherein the first side of the body is parallel to thesecond side of the body.
 8. The device of claim 1, wherein the firstbody is part of a sensor for sensing acceleration and that has fixedelectrodes that are stationary relative to the substrate, the first bodyhaving electrodes that move with the first body relative to the fixedelectrodes to cause a change in capacitance therebetween.
 9. The deviceof claim 8, wherein the sensor is part of a gyroscope that has adithered body coupled to the first body.
 10. A micromachined devicecomprising: a substrate; a first body suspended in a plane over thesubstrate and movable in the plane along a first direction, the firstbody having a substantially rectangular perimeter portion; an anchoredmember, elongated along a side of the substantially rectangular firstbody along a second direction transverse to the first direction, theanchored member having first and second opposite ends in the lengthwisedirection and having flexures extending from each end of the anchoredmember to the first body, the flexures allowing the first body to movein the first direction more easily than in the second direction, themember being anchored with at least one anchor positioned centrallyalong the anchored member between the first and second opposite ends.11. The device of claim 10, further comprising a set of fingers fixedrelative to the substrate, the first body including a plurality offingers extending in the second direction and interdigitating with thefixed fingers such that movement of the fingers on the first body causesa change in capacitance between the plurality of fingers on the firstbody and the fixed fingers.
 12. The device of claim 10, furthercomprising a package, wherein the device is packaged in the packageunder ambient conditions.
 13. The device of claim 10, wherein the firstbody is symmetric about a center line parallel to the first directionand includes sides that extend generally along the first direction, theflexures being coupled to the sides of the first body, the attachmentpoints between the flexures and the anchored member being closer to thesides than to the center line.
 14. The device of claim 10, wherein thefirst body has fingers that are movable with the first body, the devicefurther comprising stationary fingers that form a differential capacitorwith the moving fingers.
 15. The device of claim 10, wherein the firstbody is part of a sensor for sensing acceleration and that has fixedelectrodes that are stationary relative to the substrate, the first bodyhaving electrodes that move with the first body relative to the fixedelectrodes to cause a change in capacitance therebetween.
 16. The deviceof claim 15, wherein the sensor is part of a gyroscope that has adithered body coupled to the first body.
 17. A micromachined devicecomprising: a substrate; a first body suspended in a plane over thesubstrate and movable in the plane along a first direction, the firstbody having a substantially rectangular perimeter portion; an anchoredmember, elongated along a first side of the substantially rectangularperimeter portion, and including multiple anchors with one near the sidethe anchor is next to along a second direction transverse to the firstdirection, the anchored member first and second opposite ends in thelengthwise direction and having flexures extending from each end of theanchored member to the first body, the flexures allowing the first bodyto move in the first direction more easily than in the second direction,the member being anchored with at least one anchor positioned centrallyalong the anchored member between the first and second opposite ends.18. The device of claim 17, wherein one of the anchors is near an edgeof the anchored member facing the first body.
 19. The device of claim17, further comprising a second anchored member elongated along a secondside of the substantially rectangular perimeter portion, the secondanchored member having first and second opposite ends in the lengthwisedirection and having flexures extending from each end of the anchoredmember to the first body, the flexures allowing the first body to movein the first direction more easily than in the second direction, themember being anchored with at least one anchor positioned centrallyalong the anchored member between the first and second opposite ends.20. The device of claim 19, wherein the first side of the body isparallel to the second side of the body.
 21. The device of claim 17,wherein the first body is part of a sensor for sensing acceleration andthat has fixed electrodes that are stationary relative to the substrate,the first body having electrodes that move with the first body relativeto the fixed electrodes to cause a change in capacitance therebetween.22. The device of claim 21, wherein the sensor is part of a gyroscopethat has a dithered body coupled to the first body.